Document 1 (JP 2008-11229 A) discloses a multicast network system which includes a monitoring server and a plurality of response agents (terminals). The monitoring server creates a response request message including a response probability, and therefore sends the response request message to the plurality of the response agents. The response agent judges, on the basis of the response probability, whether or not it responds to the response request message. The response agent sends a response message (response signal) upon determining that it responds to the response request message.
In the network system disclosed in Document 1, some response agents receiving the response request message sends the response message to the monitoring server. Consequently, the number of response messages sent to the monitoring server can be less than the maximum number of the response messages which the monitoring server receives simultaneously.
However, the network system disclosed in Document 1 cannot receive the response messages respectively from all of the response agents.